The present invention relates to the economical conversion of relatively low-value coal and gypsum to valuable gas streams and solid products. More particularly, the invention relates to a process for the coproduction of a combustible gas stream usable as an energy source and a sulfur-containing gas stream usable as a feedstock for the production of sulfuric acid. An additional useful by-product of the process is a sintered solid inorganic product that may be used as an aggregate for paving materials as well as other uses.
Natural phosphate rock, particularly the mineral, apatite (calcium phosphate), is a primary commercial source of phosphorus. One of the most common methods of producing phosphoric acid from phosphate rock is the acid or wet process. The wet process comprises digesting refined phosphate rock with sulfuric acid to produce phosphoric acid and an impure calcium sulfate, known as phosphogypsum. Phosphogypsum has, until recently, been considered a waste product of the wet process, having no commercial value. Thus great mounds of phosphogypsum have accumulated near and around phosphoric acid plants. These mounds of phosphogypsum pose an environmental problem due mainly to the acidulation of rainwater runoff from the soluble compounds in the phosphogypsum.
One commercially valuable process for the conversion of phosphogypsum into useful products is disclosed in U.S. Pat. No. 4,503,018 issued to Gardner et al. ("Gardner"), which is incorporated herein by reference in its entirety. The Gardner process yields a sulfur-containing gas stream resulting from the thermal decomposition of the gypsum. More specifically, the Gardner process involves charging a pelletized mixture of carbonaceous material and gypsum to a travelling grate where the mixture is dried and heated to produce a gaseous effluent containing sulfur dioxide and/or sulfur. After the pellets have undergone thermal decomposition, the lime residue may be sold or used in conventional applications.
The gypsum desulfurization process taught by Gardner is, overall, endothermic. The process therefore employs an external source of heat for the thermal decomposition of the gypsum. In one embodiment, the Gardner process utilizes a coal gas producer operating on a high-sulfur coal to produce a hot, raw, low-BTU gas. The coal gas producer thus supplies heated gas to the travelling grate reactor. Although Gardner contemplates recovering heat from the product gas, the process does not yield significant amounts of energy for export.
The optional coal gas producer used in the Gardner process is one which converts substantially all of the carbonaceous and sulfurous compounds in the coal to volatile gases which are fed, without pretreatment, directly to the travelling grate reactor. The carbon from the coal feedstock therefore functions largely as a source of heat and does not participate significantly in the chemical reduction of the sulfur compounds in the gypsum. A substantial portion of the sulfur from the coal feedstock emanates from the coal gas producer in the form of hydrogen sulfide. Certain disadvantages have attended feeding high-hydrogen sulfide content gases directly to the travelling grate reactor. For example, when such gases are combusted directly, they lead to higher water content and lower SO.sub.2 content in the gaseous product than when solid sulfur-containing materials are fed to the reactor.
The Gardner process is a valuable process for converting phosphogypsum into usable products, and those skilled in the art are continuously striving to find methods of improving the efficiency of desulfurization of gypsum and upgrading the quality of the products of that process.